This invention relates to a method of preparation of a boratabenzene containing metal complex.
The usefulness of olefin polymerization catalysts containing a transition metal pi-bonded to a ligand that contains a boratabenzene ring has recently been realized. Such catalysts may be used in the homo- and co-polymerization of ethylene and other olefinic hydrocarbons. These catalysts offer several advantages over the conventional Ziegler catalyst systems which contain a transition metal and one or more organometallic compounds. Specifically, the boratabenzene-containing catalysts have exhibited higher activities, thereby making it feasible to use a lesser amount of the catalyst. Lower concentrations of the boratabenzene catalysts have made it less important to remove catalyst residues. Conventional catalysts have required that neutralizing agents and stabilizers be added to the polymers to overcome the deleterious effects of the catalyst residues. Failure to remove residues results in polymers having a yellow or grey color and poor ultraviolet and long term stability. Chloride-containing Ziegler catalysts can cause corrosion in polymer processing equipment. Ziegler catalysts tend to produce polymers with a broad molecular weight distribution, which is undesirable for injection molding applications. Furthermore, Ziegler catalysts are not very efficient at incorporating xcex1-olefin co-monomers, thereby making it difficult to control polymer density.
Although Ziegler catalysts have been improved, these catalysts are being replaced with metallocene catalyst systems. A metallocene consists of a transition metal with one or more cyclopentadienyl ligands attached. The metallocene catalysts have low activities when used with organometallic co-catalysts such as aluminum alkyls, but have high activities when used with aluminoxanes as co-catalysts. Activities are so high that it is not necessary to remove the catalyst residue from the polymer. These catalysts also incorporate xcex1-olefins well. However, at high temperature they tend to produce lower molecular weight polymers. As such, they are most useful for gas phase and slurry polymerizations of ethylene which are typically conducted between 80 and 95xc2x0 C. The improved co-polymerization of ethylene is desirable because it allows greater flexibility for producing polymers over a wider range of densities as well.
Relatively few synthetic routes to boratabenzene-containing compounds are known. An early route was the hydrostannation of 1,4-pentadiyne with dibutylstannane to give boracyclohexadiene on exchange with boron halides. Boracyclohexadiene is then deprotonated with a base such as lithium diisopropylamide (LDA) to give lithium boratabenzene. Another route for preparing boratabenzene is based on the metalation induced ring closure of [bis(dialkylamino)-boryl]pentadienes.
It is an object of the present invention to provide an improved method of preparing a boratabenzene-containing metal complex.
It is another object of the present invention to provide a method of preparing a boratabenzene-containing metal complex useful as an olefin polymerization catalyst.
The present invention provides a method for preparing a boratabenzene-containing complex. In one embodiment of the invention, the preparation of the boratabenzene-containing complex comprises the hydrogenation of a compound that contains a boranaphthalene functional group. The invention describes two distinct types of compounds that are hydrogenated as part of a synthetic route to a boratabenzene-containing complex. One type of compound that may be hydrogenated is a Group 1 or Group 2 metal salt of the boranaphthalene ligand. A second type of compound that may be hydrogenated is a transition metal complex containing a boranaphthalene ligand. This transition metal containing a boranaphthalene ligand may itself be a catalyst.
In accordance with a second embodiment of the present invention, a method for preparing a boratabenzene-containing complex is provided. In this embodiment, a pentadienyl-dioxaborolane is prepared by reacting a piperylide salt with a halo-dioxaborolane. The pentadienyl-dioxaborolane thus formed is further reacted with a strong base such as lithium diisopropylamide (LDA) to form an intermediate boratacyclohexadiene salt. This intermediate salt is subsequently treated with a trialkylaluminum to form an alkyl boratabenzene salt.
Reference will now be made in detail to presently preferred embodiments and methods of the invention, which constitute the best modes of practicing the invention presently known to the inventor.
In accordance with one embodiment of the invention, a presently preferred method of preparing a boratabenzene complex is provided. The method of the present invention comprises hydrogenation of a compound having a substituted boranaphthalene group or ligand to form a boratabenzene complex having the following formula: 
where M is a metal selected from Groups 4 to 10 of the Periodic Table; n is such that the complex I is neutral; X is a halogen, a C1-8 alkyl group, a C6-10 aryl group, a C7-15 aralkyl group, C1-10 alkoxy, or C1-10 dialkylamino group each of these groups optionally substituted with a halogen, a cyano group, a C1-4 alkoxy group, or a C1-4 alkyl group; L1 is a hydrogenated boranaphthalene ligand with the structure: 
where R1 is hydrogen, a C1-8 alkyl group, C6-10 aryl group, C7-15 aralkyl group, C1-10 alkoxy group, C6-14 aryloxy group, C1-8 dialkylamino group, or C6-15 diarylamino group; R2 is a C1-8 alkyl group or a C6-10 aryl group; m is 0 to 4; and L2 is a second hydrogenated boranaphthalene ligand, a boratabenzene ligand, or an optionally substituted cyclopentadienyl ring. The method of the present invention comprises the hydrogenation of a compound containing the following boranaphthalene functional group: 
Hydrogenation is accomplished with hydrogen gas in the presence of a suitable catalyst. Suitable hydrogenation catalysts are chosen from Group 9 and 10 metals. The catalyst is typically a metal which is highly divided or dispersed evenly over the surface of an inert carrier. Such carriers include but are not limited to carbon, alumina, kieselguhr, and Celite. Oxides of Group 9 and 10 metals may also be used as catalysts. However, the oxide will be reduced during the hydrogenation and produce water as a by-product. The water in turn will react with the complex being hydrogenated to produce a hydroxylated form of the polymerization catalyst. Many of these are known to have low polymerization activity, so the choice of hydrogenation catalyst must take this into consideration. The hydrogenation is accomplished with the compound to be reduced dissolved in a non-reactive solvent such as tetrahydrofuran (THF), toluene, or methylene chloride. The preferred catalyst is palladium over carbon.
In accordance with one aspect of this embodiment, the compound containing the boranaphthalene functional group is: 
where L3 is the boranaphthalene functional group given in structure III, L4 is a second boranaphthalene functional group or an optionally substituted cyclopentadienyl ring, X is a halogen, and n is such that compound IV is neutral. Compound IV may be prepared by reaction of a complexing, xcfx80-bonding cyclic ligand anion L4xe2x8ax96 A⊕ VII with a boranaphthalene metal complex of the structure: 
In accordance with another embodiment of the invention, a presently preferred method of preparing a boratabenzene complex is provided. The method of the present invention comprises hydrogenation of a finished catalyst with formula: 
preferably of the formula: 
where A⊕ is the cation of a strong base, preferably Lixe2x8ax96, and
where R3 is an alkyl group containing 1 to 15 carbon atoms. The hydrogenation of these compounds from boratabenzene salts with the following formulae: 
respectively.
Suitable hydrogenation catalysts are the same as those described above. Compound XI is reacted with a transition metal compound of the formula: 
preferably CpZrCl3 (Cp=cyclopentadienyl group) to form a boratabenzene complex with the formula: 
In accordance with another embodiment of the invention, a presently preferred method of preparing a boratabenzene complex is provided. The method of the present invention comprises hydrogenation of a finished catalyst with formula: 
where R4 is an alkyl group containing 1 to 15 carbon atoms. Compound XIII is hydrogenated with H2 over a palladium catalyst to form: 
In accordance with another embodiment of the invention, a presently preferred method of preparing a boratabenzene complex is provided. The method of the present invention comprises hydrogenation of an intermediate transition metal complex with the formula: 
preferably one of the formula: 
where R5 is an alkyl group containing 1 to 15 carbon atoms. The preferred alkyl groups are t-butyl and isopropyl. The product resulting from the hydrogenation is a compound of the structure: 
Compound XV is formed by reacting compound X with a transition metal halide such as ZrCl4. Hydrogenation of compound XV forms: 
Compound XVI is next treated with lithium 1-methylboratabenzene to form: 
Alternatively, compound XVI is treated with sodium cyclopentadienide (NaCp) to form: 
In accordance with still another aspect of the invention, a method for preparing a boratabenzene derivative is provided. The boratabenzene derivative of the present embodiment has the following structure: 
where R6 is an alkyl group containing 1 to 15 carbon atoms. The method of this embodiment comprises the formation of a pentadienyl-dioxaborolane by reacting potassium piperylide with a halo-dioxaborolane with the following formulae: 
where R is an alkyl group having 1 to 15 carbon atoms and X is a halogen; 
where R7 is an alkyl group having 1 to 15 carbon atoms and X is a halogen; or 
where n is from 1 to 10 and X is a halogen.
The pentadienyl-dioxaborolanes formed from compounds XX, XXI, and XXII are each further reacted with a strong base such as lithium diisopropylamide (LDA) to form an intermediate boratacyclohexanediene salt. This intermediate salt is subsequently treated with a trialkylaluminum to form an alkyl boratabenzene salt.
The pentadienyl-dioxaborolane formed by reacting potassium piperylide with compound XX has the following formula: 
Compound XXII is then treated with lithium diisopropylamide (LDA) to form an intermediate boratacyclohexanediene salt with formula: 
Compound XXIV is then treated with trimethylaluminum at xe2x88x9278xc2x0 C. to form lithium 1-methylboratabenzene.